It is clear that virus-specific CTL are critically important in limiting lentiviral replication. Despite the importance of CTL, T lymphocyte responses ultimately fail to control HIV-1 replication and progression to AIDS and death results. Protective vaccines will undoubtedly need to induce CTL, but development of such vaccines is hampered because the correlates of T lymphocyte-mediated protection are not known. Specific knowledge gaps include how to induce and maintain protective CTL in an MHC class I disparate population, the specific epitopes recognized by protective CTL, the qualitative characteristics (i.e., functional avidity) of protective CTL, and whether protection can occur in the absence of neutralizing antibody. [unreadable] [unreadable] The overall goal of the proposed research is to develop immunization methods that induce protective anti-lentiviral CTL responses in individuals with diverse MHC class I backgrounds. The lentiviral system under study is EIAV in horses. Most horses control EIAV replication within a year and remain persistently infected inapparent carriers. In immunocompetent horses, CTL can be demonstrated concurrent with clearance of the initial viremia, prior to the appearance of neutralizing antibody, indicating that CTL are critical in EIAV control. ElAV-specific CTL epitopes have been identified in Gag, Pol, Env, Rev, and S2, with Gag-specific CTL occurring in a high percentage of infected horses. Importantly, four CTL epitope clusters occur in the p15 matrix and p26 capsid proteins of Gag. These epitope clusters contain conserved epitopes that are recognized by high avidity CTL from inapparent carrier horses with diverse MHC class I alleles. [unreadable] [unreadable] The experiments outlined in this proposal will determine if a DNA vaccine encoding conserved Gag-specific epitope clusters, augmented with a plasmid expressing an equine IL-2/lgG fusion protein, will induce high avidity Gag-specific CTL in horses with diverse MHC class I alleles. Additional experiments will determine whether or not these horses are subsequently protected against EIAV challenge. This research is consistent with the exploratory/developmental nature of the R21 mechanism as described in PA-03-082 because it evaluates a novel strategy to induce protective anti-lentiviral CTL in MHC class I disparate individuals using a nucleic acid vaccine. Because our vaccine constructs will not encode envelope proteins, protective effects will occur in the absence of neutralizing antibody. Accomplishing these aims should help define the correlates for CTL-induced protection against lentivirus challenge in a diverse population. The information obtained from the proposed studies should have implications for HIV-1 vaccine design, where experiments of this type may be more difficult. [unreadable] [unreadable]